The term "alternative energy" can be confusing - With a number of words and phrases tossed about to mean many of the same things, it can be difficult to differentiate between definitions for renewable or green energy, sustainability, alternative energy and similar terms.
One of the ways to understand what is meant by the term alternative energy is to think in terms of the various types of energy production. As much as we talk about renewable or green energy, the fact is that most energy used around the world today is still derived from fossil fuels. These can take a number of forms, be it coal, natural gas or oil. One way that alternative energy can be defined is to state that the term refers only to energy that is derived from a source other than alternative fuels. This definition, however, is somewhat limiting in that it would not include vehicles field by natural gas. Defining Alternative
This has led to a broader definition in which alternative energy is being used as a term to describe energy sources other than petroleum based. This broader definition allows for inclusion of natural gas fueled vehicles and also eliminates debate over whether electric cars should be considered alternative energy vehicles.
Regardless of whether a very specific definition or a broader one is used, most think of alternative energy as a term used to describe energy--whether from fossil fuels or not--as those that are greener that traditional petroleum-based fuels. But is this true? That's a question that continues to be raised as the alternative energy portfolio has continued to grow. Renewable Versus Alternative Energy
If alternative energy is defined strictly as a renewable energy, it follows that it is also a non-polluting source with minimal environmental impact. However, alternative energy sources can raise environmental problems as well. In addition, the process for making use of alternative fuels can have environmental impact that falls along a broad impact spectrum. A simple example is wood. Certainly by any definition, wood is thought of as a renewable and alternative energy source. However, burning wood causes pollution if it is not done properly. Therefore, wood itself remains a green energy source, but the process for converting wood to energy may not necessarily be green.
Another example is natural gas, which is also commonly considered as an alternative fuel. In defining natural gas as an alternative energy, we are unable to make the leap that if it is alternative, it must also be renewable. This has become a bit of a stumbling block at times as natural gas proponents can sometimes talk as though natural gas were renewable, which leads to some confusion over the whole alternative and renewable definitions again.
Another way to think of renewable energy is in terms of an energy source that is derived from the sun, whether directly or indirectly. By this definition, An obvious example of renewable energy, or energy derived from the sun, is sunlight itself, in the form of solar energy, which is used for lighting and heating commercial and residential buildings, as well as for generating energy to be used for a number of tasks, such as heating water or solar cooling. Those solar panels, for example, once unique, have now started popping up with more regularity.
But the sun has an indirect affect on renewable energy as well. For example, many people understand wind energy, but it is actually the sun's heat that drives the wind and then wind energy that is then captured with wind turbines.
Taken a step further, the winds and the heat from the sun will cause water to evaporate. This water vapor can then turns into rain or snow. As it accumulate and flows downhill into rivers or streams, its energy can be harnessed again, this time in terms of hydroelectric power.
Want another example? Along with the rain and snow, sunlight is necessary for plants to grow. Those plants are made out of organic matter, which is also known as biomass. There is a great deal of renewable energy development targeted toward biomass which can be used to make electricity, vehicle fuels, or even chemicals. The use of biomass for any of these purposes is called bioenergy.
source : about.com